The present disclosure relates generally to a method for forming ceramic powders, and more particularly to a flame spray pyrolysis method for forming lithium metal phosphate ceramic powders. Such powders can have nanoscale particle sizes and can be used to form lithium ion-conductive ceramic membranes. For instance, as disclosed herein, effective Li ion-conductive ceramics include lithium aluminum titanium phosphate (LATP), which can include materials having the nominal composition Li1.4Al0.4Ti1.6(PO4)3, and lithium aluminum germanium phosphate (LAGP), which can include materials having the nominal composition Li1.4Al0.4Ge1.6(PO4)3.
Powders of suitable ceramic compositions can be cast and sintered to form physically and chemically-durable membranes. Lithium ion-conductive membranes can be used in Li-air and Li-seawater batteries, for example, where the membrane separates air or water from the metallic lithium metal anode inside the cell. Ceramic Li ion-conductive membrane can also be used in a complete solid state Li-ion battery as a substitute for the polymeric membrane.
Conventionally, LATP and LAGP membranes are made by a glass ceramic route where related precursors (e.g., oxides, carbides, NH4H2PO4, etc.) are mixed and melted at elevated temperatures (e.g., 1500° C.) and then quenched. The resulting glass is milled into a powder, which can typically have micro-scale particle sizes. The powder is heat treated to crystallize it into a glass ceramic. The glass ceramic can be tape-cast to form a film that, in turn, is heated (e.g., to 800-900° C.) and sintered to form a dense membrane. With this glass ceramic approach, however, a high melting temperature is needed to melt the precursor materials, and the quenching step has to be precisely controlled to form the glass phase.
Glass phase LATP and LAGP powders can also be made by a sol-gel process. The resulting powder can be crystallized and formed into a dense membrane, though this method can be difficult to scale-up. A further method for making crystallized LATP powders is a co-precipitation method. The co-precipitation method can be used to directly tape cast a sinterable film. However, the co-precipitation method generally results in particle sizes that are too large for membrane durability in sea water applications.
Flame spray pyrolysis (FSP) may be used to form powder materials from chemical precursors, though the FSP approach may be difficult to apply to precursors that contain highly-volatile constituents such as lithium due to the high temperature conditions in the flame and the attendant difficult in retaining the volatile components in the final product. In view of the foregoing, it would be beneficial to develop a flame spray pyrolysis process for forming stoichiometric (non-lithium deficient) LATP and LAGP nanoscale powders. Such powders can be conductive to lithium ions and can be incorporated into ceramic membranes for advanced lithium battery applications for example. As disclosed herein, the LATP and LAGP powders can be formed with a small (<50 nm diameter) particle size, and have been shown to be effective sintering aids in forming LATP or LAGP membranes.